The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve where the resistance increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are illustrated in FIG. 1. They are seed layer 11 on which is antiferromagnetic layer 12 whose purpose is to act as a pinning agent for a magnetically pinned layer. The latter is a synthetic antiferromagnet formed by sandwiching antiferromagnetic coupling layer 14 between two antiparallel ferromagnetic layers 13 (AP2) and 15 (AP1).
Next is a copper spacer layer 16 on which is low coercivity (free) ferromagnetic layer 17. Capping layer 18 (a laminate of three layers 18a, 18b, and 18c which are typically Cu, Ru, and Au respectively) lies atop free layer 17. When free layer 17 is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will be at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field.
If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 8–20%.
Also seen in FIG. 1 are the two magnetic shields 8 and 9 (S1 and S2) which are typically each about 2 microns thick.
Most GMR devices have been designed so as to measure the resistance of the free layer for current flowing parallel to its two surfaces (CIP). However, as the quest for ever greater densities has progressed, devices that measure current flowing perpendicular to the plane (CPP) have begun to emerge. For devices depending on in-plane current, the signal strength is diluted by parallel currents flowing through the other layers of the GMR stack, so these layers should have resistivities as high as possible. In contrast, in a CPP device, the total transverse resistance of all layers, other than the free layer, should be as low as possible so that resistance changes in the free layer can dominate.
A related device to the CPP GMR described above is the magnetic tunneling junction (MTJ) in which the layer that separates the free and pinned layers is a non-magnetic insulator, such as alumina or silica. Its thickness needs to be such that it will transmit a significant tunneling current. The principle governing the operation of the MTJ in magnetic read sensors is the change of resistivity of the tunnel junction between two ferromagnetic layers when it is subjected to a bit field from magnetic media. When the magnetizations of the pinned and free layers are in opposite directions, the tunneling resistance increases due to a reduction in the tunneling probability. The change of resistance is typically 40%, which is much larger than for GMR devices.
Since the free layer thickness is limited (typically about 30 Å), the AP1 layer has to be thicker for a higher CPP GMR. As a result, AP2 must also be thicker because its magnetic moment has to match that of AP1 for effective pinning to occur. A typical sample configuration might be as follows:(S1)/Ta5/NiCr50/IrMn70/FeCo10/CoFe50/Ru4/FeCo60/Cu30/CoFe30/cap/(S2)
where S1 and S2 are bottom and top magnetic shields, respectively and the various thicknesses associated with each layer are in Angstroms.
Because of the increase of the AP1 and AP2 thicknesses, the free layer gets shifted further away from the center of the read gap, an undesirable situation.
For better power dissipation, thermoelectric cooling (TEC) leads 21 and 22 may be inserted on top of the bottom shield and below the top shield, respectively, as shown in FIG. 2. This causes the read gap to be further widened, another undesirable situation.
A key feature of the present invention is a method to reduce the gap width as well as to shift the free layer closer to the center of the read gap.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 5,668,688 Dykes et al. describe NiFe shield layers and in U.S. Pat. No. 6,563,679 Li et al disclose a structure that we describe above. Li et al. teach a metal pillar carrying sense current connected to the top or bottom shield, in U.S. Pat. No. 6,512,660 while Pang et al. shows an extra shield for a sensor element in U.S. Pat. No. 6,496,334. Also of interest is M. Lederman et al (U.S. Pat. No. 5,627,704).